In a telecommunications system such as LTE (Long Term Evolution) OFDM (Orthogonal Frequency Division Multiplexing) is used in the downlink and DFT (Discrete Fourier Transform)-spread OFDM (a.k.a. Single-Carrier Frequency Division Multiple Access, SC-FDMA) in the uplink. The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1 where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising of ten equally-sized subframes of length Tsubframe=1 ms (see FIG. 2).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information indicating to which terminals and on which resource blocks the data is transmitted during the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as control region is illustrated in FIG. 3.
In LTE, a plurality of channels is defined such as the Physical Uplink Control Channel (PUCCH) and the Physical Uplink Shared Channel (PUSCH) which are described below.
For PUCCH LTE is configured to use hybrid-ARQ (hybrid Automatic Repeat reQuest), where, after receiving downlink data in a subframe, the mobile terminal or often referred to as the User Equipment (UE) attempts to decode it and reports to the base station whether the decoding was successful Acknowledgment (ACK) or not (NACK) (Non-Acknowledgment)). In case of an unsuccessful decoding attempt, the base station may retransmit the erroneous data.
Uplink control signaling from the terminal (e.g. UE) to the base station may include:
hybrid-ARQ acknowledgements for received downlink data;
terminal reports related to the downlink channel conditions, used as assistance for at least the downlink scheduling (also known as Channel Quality Indicator (CQI));
scheduling requests, indicating that the terminal needs uplink resources for uplink data transmissions.
If the mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 (Layer 1 and/or Layer 2) control information (channel-status reports, hybrid-ARQ acknowledgments, and scheduling requests) is transmitted in uplink resources (resource blocks) assigned for uplink L1/L2 control information on the PUCCH. Different PUCCH formats are used for different information. For example, PUCCH Format 1a/1b and 3 are used for hybrid-ARQ feedback, PUCCH Format 2/2a/2b for reporting of channel conditions, and PUCCH Format 1 for scheduling requests. The different PUCCH formats are described in the Third Generation Partnership Project LTE standard related Technical Specifications 3GPP TS 36.213.
To transmit data in the uplink the mobile terminal has to be assigned an uplink resource for data transmission, on the Physical Uplink Shared Channel (PUSCH), see FIG. 4.
The middle single-carrier symbol in each slot is used to transmit a reference symbol. If the mobile terminal has been assigned an uplink resource for data transmission and at the same time instance has control information to transmit, it will transmit the control information together with the data on PUSCH.
LTE Rel-8 (Release-8) has recently been standardized, supporting bandwidths up to 20 MHz. However, in order to meet the International Mobile Telecommunications Advanced (IMT-Advanced) requirements, 3GPP has initiated work on LTE Rel-10. One of the key components of LTE Rel-10 is the support of bandwidths beyond 20 MHz while ensuring backward compatibility with LTE Rel-8. This should also include spectrum compatibility and implies that an LTE Rel-10 carrier, potentially wider than 20 MHz may be realized as a number of LTE carriers to an LTE Rel-8 terminal (e.g. a UE). Each such carrier may be referred to as a Component Carrier (CC). For early LTE Rel-10 deployments it may be expected that there will be a smaller number of LTE Rel-10 capable terminals compared to many LTE legacy terminals. Therefore, it may be necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals may be scheduled in all parts of the wideband LTE Rel-10 carrier. A way to obtain this would be by means of Carrier Aggregation (CA) so that that an LTE Rel-10 terminal may receive multiple CCs, where a CC has, or at least have the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5.
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It should be noted that the number of CCs configured in a cell area may be different from the number of CCs seen or used by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the network is configured with the same number of uplink and downlink CCs.
CCs are also referred to as cells or/and serving cells. In an LTE network the component carriers aggregated by a terminal generally are denoted primary cell (PCell) and secondary cells (SCells). The term Serving Cell comprises both PCell and SCells. The PCell is terminal specific and is considered “more important”, due to that control signaling and other important signaling is typically handled via the PCell. The CC configured as the PCell is the primary CC whereas all other component carriers are secondary CCs.
LTE also supports uplink power control. Uplink power control is used both on the PUSCH and on the PUCCH. The purpose is to enable that the mobile terminal transmits with sufficiently high power but not too high power since the latter may increase the interference to other users in the network. In both cases, a parameterized open loop combined with a closed loop mechanism may be used. Roughly, the open loop part is used to set a point of operation, around which the closed loop component operates. Different parameters (targets and ‘partial compensation factors’) for user and control plane may be used. For further description of PUSCH and PUCCH power control, see sections 5.1.1.1 and 5.1.2.1 respectively of 3GPP TS 36.213, Physical Layer Procedures
In order to control the mobile terminal's or the UE's uplink (UL) power the base station or eNB also called evolved Node B uses TPC (Transmission Power Control) commands to order the UE to change its transmission power either in an accumulated or absolute fashion. In LTE Rel-10 the UL power control is managed per CC. As in Rel-8/9 PUSCH and PUCCH power control is separate. In LTE REI-10 the PUCCH power control will only apply to the Primary CC since this is the only UL CC configured to carry PUCCH.
It should be noted that since the TPC commands do not have any ACK/NACK bits, the eNB cannot be sure that they are received by the UE, and since the UE may falsely decode the Physical Downlink Control Channel (PDCCH) and think/assume it received a TPC command, counting the used TPC commands may not be used to estimate a reliable current output power from the UE. The UE may also compensates its power level autonomously (based on pathloss estimates) and this adjustment is not known to the eNB base station. For these two reasons the eNB base station may need to receive PHR (Power Headroom Report) reports regularly in order to make competent scheduling decisions and control the UE UL power.
In e.g. Rel-8/9 the eNB base station configures the maximum output power of the UE. Since the UE is allowed to make power backoffs, the actual transmission power the UE is capable of may deviate from the configured power. The UE is configured or adapted to select a value, here denoted, Pcmax which is the actual maximum transmission power after power backoff and which may be used to calculate the power headroom left in the UE.
The UE is allowed to backoff its transmission power to ensure that out-of-band emissions do not exceed the specified maximum values. This backoff operation may also be used in other releases or other systems/technologies as well to ensure that out-of-bands emissions do not exceed specified maximum values. The corresponding allowed power reduction is referred to as MPR (Maximum Power Reduction) and A-MPR (Additional-MPR). The UE is allowed to back off its transmission power by up to the defined MPR+A-MPR value, but the UE is not required to back off as long as it meets the inter-band emission requirements. The maximum power reduction allowed for a UE in a specific deployment using a specific MCS (Modulation and Coding Scheme) and number of resource blocks is e.g. defined in tables in 3GPP TS 36.101. The tables do however only define the maximum allowed values and the eNB base station does thus not know the exact value of the applied MPR/A-MPR.
In Rel-10, the eNB base station configures the maximum output power of each CC individually. Similar to how Pcmax was selected by the UE in Rel-8/9, the UE selects a Pcmax,c for each CC which it uses to calculate the power headroom for the associated CC.
Pcmax,c is the actual maximum transmission power for a specific CC configured by the UE in a specific TTI (Transmission Time Interval). It is set within an interval with the upper boundary defined by the maximum value of the UE power class and the maximum CC power configured by the eNB base station, and the lower boundary defined by taking maximum power reduction (MPR/A-MPR) into consideration.
In addition to power backoff to meet out-of-band emissions, the UE is also configured/adapted to fulfill SAR (Specific Absorption Rate) requirements which also may require the UE to back off its transmission power (possibly in addition to the backoffs made to meet the out-of-band emissions). This may be the case when the UE supports both LTE and WCDMA (Wideband Code Division Multiple Access) technologies and operates both radio access technologies simultaneously. It has therefore been agreed in 3GPP that LTE Rel-10 UEs may perform an additional power back-off for so-called “power management” purposes. This comprises but is not limited to SAR related power back-off. It has also been decided that the UE shall, when performing such a power back-off, reflect it in the computation of Pcmax and/or Pcmax,c.
Consequently, this additional power reduction will be reflected in the Pcmax and/or Pcmax,c report as well as in the PHR reports (see following section).
As previously described, in LTE Rel-8, the base station may configure the UE to send PHR reports periodically or when the change in pathloss exceeds a certain configurable and/or predetermined threshold. The PHR reports indicate how much transmission power the UE has left for a subframe I, i.e., the difference between the actual UE maximum transmit power (Pcmax,c or Pcmax) and the estimated required power. The reported value is in the range of 40 to −23 dB, where a negative value shows that the UE did not have enough power to conduct the transmission.
The eNB base station may use the PHR report as input to the scheduler. As an example, based on the available power headroom the scheduler of the eNB base station is configured to decide a suitable number of PRBs (Physical Resource Blocks) and a good/suitable/adequate MCS as well as a suitable transmit power adjustment (TPC command). In carrier aggregation the eNB base station would make such evaluation per UL CC because power is controlled per CC according to RAN1 decisions.
Since one has UL power control per CC and separate for PUSCH and PUCCH, this will also be reflected in the PHR reporting. In Rel-10, the UE will calculate one Pcmax,c value per CC and also calculate a separate power headroom per CC. For Rel-10 at least two types of power PHR reports may be used:
Type 1 PHR report—computed as: P_cmax,c minus PUSCH power: (P_cmax,c−P_PUSCH)
Type 2 PHR report—computed as: P_cmax,c minus PUCCH power minus PUSCH power: (P_cmax,c−P_PUCCH−P_PUSCH)
The Secondary CCs may always report Type 1 PHR report since they are not configured for PUCCH. The Primary CC may report both Type 1 and Type 2 PHR report. Type 1 and Type 2 PHR report may be reported in the same subframe or in separate subframes.
In Rel-10, a PHR report for one CC may be transmitted on another CC. This would enable to report rapid pathloss changes on one or more CCs as soon as the terminal has PUSCH resources granted on any activated UL CC. More specifically, a pathloss change by more than dl-PathlossChange dB on any activated CC may trigger PHR reports for all activated CCs, independent whether a valid PUSCH grant is available or not for a CC. All PHR reports may be transmitted together in the same MAC control element (CE) in the same subframe on the same CC. This CC may be any CC for which the terminal has PUSCH resources granted.
In Rel-10, all PHRs to be reported in a specific subframe may be included in the same MAC CE and transmitted on one of the activated UL CCs. There is at the most one PHR report or Extended PHR MAC CE per TTI.
In addition to the PHR report, there may be a Pcmax,c report per CC reporting the actual maximum transmission power of the UE, denoted Pcmax,c in 3GPP TS 36.213. As explained before, the Pcmax,c value is affected by UE power reductions due to out-of-band emissions requirements (MPR/A-MPR) or SAR (power management) requirements. The Pcmax,c value being sent in addition to the PHR value enables the network or network nodes (e.g. base station) to estimate the reason for the change in power headroom, i.e., whether it was due to a change in the available transmit power (Pcmax,c) or due to a change in pathloss and TPC command errors.
In Rel-10, the Pcmax,c is included in the same Extended PHR MAC CE as the associated PHR.
The Extended PHR MAC (Medium Access Control) CE is defined in Rel-10 of 3GPP TS 36.321. An example of the structure is shown in FIG. 6. for definitions of included fields refer to 3GPP TS 36.321. The acronyms used e.g. R, V are also defined in 3GPP TS 36.321.
PHR may, in Rel-10, be reported for all configured and activated CCs. This means that some of the CCs reporting PHR may not have a valid UL grant in the TTI where PHR is reported. They will then use a reference format PUSCH and/or PUCCH to report a so called virtual/reference format PHR. These reference formats are defined in 3GPP TS 36.213.
It is possible to reduce the reporting overhead by omitting the Pcmax,c reports for CCs for which no valid UL grant has been provided. It should be noted that a Pcmax,c report computed for a reference format does not necessarily comprise any new/beneficial information for the network.
In RAN2 (Radio Access network LTE layer2 radio protocols) it is currently being discussed how power reductions related to SAR (and power management in general) requirements will impact the Power Headroom reporting.
As explained earlier, for Rel-10, the additional power back-off will be included in the Pcmax,c value reported together with the associated PHR for a specific CC.
Since the Pcmax,c value will then be dependent on two unknown factors; the MPR+A-MPR and the additional power management power reduction, it may not be possible for the eNB to derive which MPR/A-MPR was used by looking at the reported Pcmax,c and associated PHR. In other words, the additional information obtained by explicitly reporting the Pcmax,c in LTE Rel-10 vanishes partly by introducing the additional power back-off.
This is problematic as it is important for the network node(s) (e.g. eNB base station) to be able to track the MPR/A-MPR behaviour in order to optimize link adaptation and scheduling. The additional power back-off adds some “noise” to the Pcmax,c reports making it even more difficult to perform the tracking of the MPR/A-MPR.
It should be noted that since SAR (power management) reduction may not always be applied, it could be useful for the eNB base station to know which PHR reports it may use to derive the MPR/A-MPR from. Currently there is no way for the eNB to know this and to guess which part of the power reduction taken into account in Pcmax,c is not feasible considering that only ranges/tables are defined in 3GPP TS36.101 as previously described.